1. Field of the Invention
The invention relates in general to a light emitting apparatus, and more particularly to a light emitting apparatus with open loop control.
2. Description of the Related Art
Products that make use of light emitting diodes (LED) are, for example, backlights of liquid crystal display (LCD) devices, lighting devices for particular uses and vehicles, etc. In 2005, LEDs had a marketing value of 6.235 billion dollars worldwide. And in 2006, it had increased to 6.873 billion dollars. Therefore, it is anticipated that products utilizing LEDs will steadily increase in number and variety in the future. At present, white LEDs are commonly used as light sources for providing white light. A known method for fabricating a white LED is to integrate several light emitting chips of different colors. Another known method is to use phosphor powders to assist an LED to emit white light. Some of the methods for fabricating a white LED are elaborated in detail below.
In the first method, three LEDs that have indium gallium aluminum phosphide (InGaAlP), gallium phosphide (GaP) and gallium nitride (GaN) are used. Different currents are provided to the three LEDs to activate them to emit a red beam, a green beam and a blue beam, respectively, so as to produce a white beam. In second method, two LEDs made from gallium nitride (GaN) and indium gallium aluminum phosphide (InGaAlP) receive different currents which activates them emit, respectively, a blue beam and a yellowish green beam, or a green beam and a red beam, both of which generate a white beam.
The above two methods are able to automatically control the chromaticity coordinate of the white light emitted by tuning the currents to adjust the spectrum of a mixed light beam. However, control circuits for the first and second methods are far more complicated and, hence, increase the manufacturing cost.
Nichia Chemical Ltd of Japan in 1996 developed a blue LED that is composed of indium gallium nitride (InGaN) and yttrium aluminum garnet phosphor. The yellow beam from the phosphor and the blue beam from the LED combine to generate a white beam.
Sumitomo Electric Industries Ltd of Japan in January of 1999 developed a white LED that is made from zinc selenide (ZnSe). A cadmium zinc selenide (CdZnSe) thin film, which is formed on a zinc selenide single chip substrate, emits a blue beam when receiving a current. Meanwhile, part of the blue beam projects onto the single chip substrate which then emits a yellow beam. The blue and yellow beams are complementary colors and combine to form a white beam. Moreover, a UV light beam can be used to excite different phosphors to generate different color beams to form a white beam.
However, the chromaticity coordinate of the white beam generated by an LED and phosphor is related to the light beams from the LED and phosphor, and cannot be compensated automatically. Thus, the white light beam cannot be made to have a constant chromaticity coordinate.